Film and DPI

How many dpi is 35mm movie film?

A film maker friend asked for stills scanned from his 35mm original movie film frames. For publicity. And the agency wanted 300 dpi. My million-dollar Spirit scanner does 4k. So it scans a 35mm frame in a size 2048x1556 or 4096x3112. So how many dpi is that?

I set about thinking. First off, dpi is dots per inch. It is a measure for printed material. As in printed on paper. Inkjet printers do 600 or more dpi. So whether you hold the printout an inch from your nose ot at arm's length there's still 600 dots in one inch. But film? How many dpi?

When working in Photoshop one often encounters a figure of 72 dpi for TV. That is arrived at by assuming a 14" TV set. Such a set shows a picture about 13" diagonal. Which by using Pythagoras gives you about 10.6 by 8 in. For a PAL 720x576 picture that's about 72 dpi. So TV is 72 dpi for a 14" TV set. For a 20" set it is 48 dpi and a 29" TV shows just 33 dpi.

So basically for non-print images dpi is not fixed, but varies by the size of the medium. So how about film? Better than 72 dpi? Can we get some 300 dpi please?

Here's my take and I may not be entirely correct. Here goes anyway...

35mm film scanners for movie production scan at 2048x1556 pixels (at 2k) or 4096x3112 pixels (at 4k). At 2k (2048x1556) a standard 35mm frame of 18x24 mm is about 2200 dpi on the film. At 4096x3112 you've got about 4400 dpi. Impressive, but noone watches film on film. They watch it projected on a screen in a theatre.

In a theatre the same 35mm frame gets blown up to a giant screen of, say, 20 feet across. Now since the number of pixels remain unchanged you have a picture 2048 pixels by 1556 pixels. At 20 feet across, that's just over 8 dpi. Even a scan of 4096x3112 (4k) yields only 17 dpi. Appalling!

But hey, you watch that screen from 50 feet away. So you're not going to see it as 8 dpi. Its like they print magazines at 300 dpi, but the same ads in a magazine are printed on hoardings not at 300 dpi, but much less, like 10 dpi or something.

So dpi is a measure for fixed size printed matter. Which can be viewed at varying distances even very close. But film and TV are not fixed and can be viewed in various sizes and distances. But never too close. So dpi doesn't apply to film at all.